<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      height: 100px;
    }
    .left {
        float: left;
        width: 200px;
        background-color: red;
    }
    .middle {
        float: left;
        width: calc(100% - 400px);
        background-color: green;
    }
    .right {
        float: right;
        width: 200px;
        background-color: blue;
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="left">左边</div>
    <div class="middle">中间</div>
    <div class="right">右边</div>
  </div>
</body>
</html>